Active implantable medical devices include a housing containing various electronic circuits and a battery, generally referred to as a generator, that is electrically and mechanically connected to one or more leads with electrodes. The leads are intended to come into contact with a tissue of the patient, e.g., the myocardium, at sites where electrical potentials can be collected (i.e., detected) and/or stimulation pulses can be applied (i.e., delivered).
MRI examination has been contraindicated for patients with an implanted cardiac pacemaker or defibrillator type of generator. Several types of problems arise under this situation:
heating near the electrodes connecting the generator to the patient's heart;
forces and torques of attraction exerted on the device immersed in the high intensity static magnetic field produced by the MRI equipment during an examination; and
unpredictable behavior of the device itself due to the exposure to the strong magnetic field.
It is an objective of the present invention to provide a solution to the above-described problems, particularly to avoid malfunction or unpredictable behavior of the device in a strong magnetic field. In this regard, it is desirable that when the device is exposed to (static and alternating) electromagnetic fields generated by MRI equipment, the behavior of the device is documented and known in advance.
In the absence of special precautions, the problems that are likely to affect the behavior of the device under an MRI examination include an erratic detection of strong static field generated by MRI equipment, especially in devices equipped with a Reed magnetic switch. A Reed magnetic switch is used to detect and respond to the presence of a permanent magnet in the vicinity of the device. A permanent magnet is normally used by a practitioner to put the device in a safe operating or “magnet” mode, for example, when using an electric scalpel, or evaluating battery depletion of the device. In a magnet mode, the stimulation frequency is generally fixed and reflects the level of battery charge. A Reed magnetic switch is designed to detect static magnetic fields of relatively low intensity, but is likely to exhibit a totally unpredictable behavior in an MRI examination environment where the magnetic fields are often thousands of times stronger than that of a permanent magnet. Problems also may include deterioration of the intrinsic performance of the device, and misinterpretation of the dynamic signals emitted by the MRI equipment by the device as cardiac signals, including, for example, an inhibition of the stimulation function by the dynamic signals emitted by the MRI equipment that are inadvertently detected by the device as cardiac signals.
Throughout the duration of an MRI examination—which can last several minutes—the device should nevertheless remain functional and provide if necessary seamless and predictable stimulation to the patient's myocardium. It is therefore desirable to have means for detecting and means for managing such a situation, providing the following functions:
indicating to the device that the patient will be subjected to an MRI examination;
inhibiting the circuits of the device that may be disturbed by the electromagnetic fields emitted by the MRI equipment; and
operating the device in a dedicated pacing mode, tailored to the patient and compatible with the electromagnetic fields emitted by the MRI equipment.
Special techniques have been proposed to detect static magnetic fields of an MRI type, with strength in the order of Tesla (typically between 0.5 and 3 T, and herein referred to as a “strong magnetic field”): In the majority of the devices that are sensitive to the presence of a permanent magnet are those devices that have a magnetic field detector that detects weak magnetic fields (e.g., a static magnetic field in the order of 1.5 mT; and herein referred to as a “weak magnetic field”), but is unable to detect the strong magnetic fields that are produced by MRI equipment. Because the magnetic fields produced by MRI equipment are up to thousands of times stronger than that produced by a permanent magnet, and are in the non linear response zone of these weak magnetic field detectors, it poses a risk that the weak magnetic field detectors will be “deaf” to the presence of a strong magnetic field.
The U.S. Patent Publication US 2007/191914 A1 describes a device in which the presence of a strong static magnetic field is detected by an analysis of the impedance of an inductive component, e.g., one of the coils of an inductive switching regulator. The presence of a strong magnetic field has the effect of saturating the core of this inductive component, causing an impedance change that can be reported to the device.
WO 2006/124481 A2 discloses another technique for detecting the presence of an MRI-type magnetic field by the measurement of the voltage sensed across a telemetry antenna and on the lead.
EP 1935450 A1 describes yet another technique of using giant magnetoresistance (GMR) sensors associated within a Wheatstone bridge. The Wheatstone bridge acts as a single mixed strong/weak field sensor. The balance of the Wheatstone bridge is more or less altered by a magnetic field, and the resulting changes from the differential voltage can be analyzed by a converter placed at the output of the bridge to give an overall estimate of the field strength.
The U.S. Patent Publication US2009/0138058 A1 provides a programming technique to place the device in a state of waiting for MRI (referred to as an MRI mode), for example, during a consultation with a cardiologist. In this state of waiting, the device operates in its standard mode of operation, but with an expectation of detecting a strong static magnetic field. In the presence of a strong magnetic field (e.g., at the beginning of an MRI examination), the device then switches to an MRI-safe mode that is compatible with a strong magnetic field for the duration of the MRI examination. Once the MRI examination is completed, the device is reset to a normal mode of operation (referred to as a normal operation mode).
Some of these various techniques detect a strong magnetic field using an additional sensor (i.e., not the sensor responsive to a weak magnetic field). If these devices do not contain a strong magnetic field sensor, it is required to redesign the hardware of the device, incurring an additional cost and creating an extra constraint on the circuits against the design requirements for miniaturizing these devices.